Perhaps the grass in your render is too saturated? I've noticed that people tend to make their colors way more intense in renders than they are in real life. It makes the image look "better" to some extent, but it can produce side effects like this.

Or, saturation aside, it could be that your materials are just too bright in general and reflect too much light. You might experiment with darker materials and brighter light sources; this will reduce the relative intensity of bounce light. This is a very common mistake. It's a good idea to get a photographer's color card that has a set of gray levels on it, like this one. If you take this outside and hold it up against the wall of your shed to compare the colors, you might be surprised at how far from white the shed actually is!

I did some research in this when I wrote my bidirectional pathtracer (a fancier kind of raytracer), and it turns out white paper has a reflectivity of somewhere around 50%.

You'd expect that to be a medium grey, but it's not. It's all relative to how bright your lightsources are.

What I ended up doing was to specify all my colors like red = (1, 0.01, 0.01) * standardReflectiity, where "standardReflectivity" is 0.5. Notice that even my "red" has a tiny bit of green and blue, since no real material is 100% black or pure in the color.

This way I can divide the light power by the standard reflectivity to get the same exposure, in case I need to change it.

geon, do you happen to have the source for that 50% figure for white paper? I'm skeptical. In my experience white paper seems to have an albedo closer to 85-90%. Materials are often less bright than you'd think, but white paper is still pretty bright.

Right on Reedbeta. You're right, my shed isn't white like I thought. So by mult all my materials by 0.5 and my lights by 2, it looks much more natural and my shed is not green now, just a little bit, but barely. Thank you all